ImageDecoder-createFrameImage.swift

func createFrameImage(at index: Int, subsamplingLevel: SubsamplingLevel = .default, decodingOptions: DecodingOptions = .default) -> CGImage? {
    guard index < frameCount else {
        return nil
    }

    let image: CGImage?
    let options: CFDictionary

    switch decodingOptions.mode {
        case .asynchronous:
            // Don't consider the subsamplingLevel when comparing the image native size with sizeForDrawing.
            guard var size = frameSize(at: index) else {
                return nil
            }

            if let sizeForDrawing = decodingOptions.sizeForDrawing {
                // See which size is smaller: the image native size or the sizeForDrawing.
                if sizeForDrawing.width * sizeForDrawing.height < size.width * size.height {
                    size = sizeForDrawing
                }
            }

            options = imageSourceAsyncOptions(sizeForDrawing: size, subsamplingLevel: subsamplingLevel)
            image = CGImageSourceCreateThumbnailAtIndex(imageSource, index, options)

        case .synchronous:
            options = imageSourceOptions(with: subsamplingLevel)
            image = CGImageSourceCreateImageAtIndex(imageSource, index, options)
    }

    // WebKit has support for xbm images but we don't

    return image
}